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Pneumonia for ABIM

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Pneumonia for the American Board of Internal Medicine Exam
Etiology
  • Community-Acquired Pneumonia (CAP):
    • Bacterial Pathogens:
    • Streptococcus pneumoniae: The most common cause of CAP. It often presents with acute onset of fever, productive cough, and pleuritic chest pain.
    • Haemophilus influenzae: Associated with CAP in patients with chronic lung disease (e.g., COPD).
    • Moraxella catarrhalis: Seen in patients with COPD and the elderly.
    • Atypical Pathogens:
    • Mycoplasma pneumoniae: Often affects younger patients and presents with a more indolent onset of nonproductive cough, low-grade fever, and extra-pulmonary symptoms (e.g., rash).
    • Legionella pneumophila: Causes severe pneumonia with high fever, gastrointestinal symptoms (diarrhea), hyponatremia, and abnormal liver function tests.
    • Chlamydophila pneumoniae: Presents with mild respiratory symptoms and a slow progression.
    • Viral Pathogens:
    • Influenza virus: A common cause of viral CAP, especially during the flu season.
    • Respiratory syncytial virus (RSV): More common in infants and the elderly.
    • SARS-CoV-2 (COVID-19): Associated with viral pneumonia and can lead to acute respiratory distress syndrome (ARDS) in severe cases.
  • Hospital-Acquired Pneumonia (HAP):
    • Occurs ≥48 hours after hospital admission, excluding cases present at admission.
    • Common pathogens include:
    • Gram-negative bacilli: e.g., Pseudomonas aeruginosa, Klebsiella pneumoniae, and Escherichia coli.
    • Staphylococcus aureus, particularly methicillin-resistant Staphylococcus aureus (MRSA).
  • Aspiration Pneumonia:
    • Occurs when oropharyngeal contents are inhaled into the lungs, often seen in patients with altered mental status, swallowing dysfunction, or gastroesophageal reflux.
    • Common pathogens include anaerobes and oral flora, such as Streptococcus anginosus.
Pathophysiology
  • Inflammatory Response:
    • Pneumonia occurs when pathogens bypass host defense mechanisms (e.g., mucociliary clearance, alveolar macrophages) and enter the alveoli, leading to an inflammatory response. Neutrophils, cytokines, and inflammatory mediators accumulate, causing consolidation of the lung parenchyma, impaired gas exchange, and clinical symptoms such as cough, fever, and dyspnea.
Pneumonia Pathophysiology
  • Lobar vs. Bronchopneumonia:
    • Lobar pneumonia: Involves consolidation of a single lobe of the lung, classically caused by S. pneumoniae.
    • Bronchopneumonia: Patchy distribution of infection throughout the lungs, commonly associated with Staphylococcus aureus, H. influenzae, and Pseudomonas aeruginosa.
Risk Factors
  • Patient Factors:
    • Advanced age (>65 years)
    • Chronic medical conditions (e.g., COPD, asthma, heart failure, diabetes mellitus)
    • Immunosuppression (e.g., HIV, corticosteroid use, chemotherapy)
    • Smoking and alcohol use disorder
  • Environmental Factors:
    • Healthcare exposure: Recent hospitalization or residence in long-term care facilities
    • Crowded environments: Increased exposure to respiratory pathogens (e.g., schools, prisons)
Clinical Features
  • Symptoms:
    • Typical Presentation (Bacterial CAP):
    • Fever: Often high-grade, with rigors and chills.
    • Cough: Initially dry, progressing to productive cough with purulent sputum (yellow, green, or blood-tinged).
    • Pleuritic chest pain: Sharp pain that worsens with inspiration or coughing.
    • Dyspnea: Shortness of breath, especially in severe cases.
    • Atypical Presentation:
    • Mycoplasma, Chlamydia, and Legionella pneumoniae often present with gradual onset, dry cough, low-grade fever, myalgias, and extrapulmonary symptoms (e.g., gastrointestinal symptoms with Legionella).
  • Physical Examination:
    • Crackles (rales): Audible over the affected lung field.
    • Dullness to percussion: Due to lung consolidation.
    • Increased tactile fremitus: Over the area of consolidation.
    • Bronchial breath sounds: With egophony (“E” to “A” change).
Diagnosis
  • Clinical Diagnosis:
    • Pneumonia is diagnosed based on clinical symptoms (fever, cough, dyspnea, pleuritic pain) and physical exam findings (rales, decreased breath sounds, dullness on percussion).
  • Imaging:
    • Chest X-ray: The gold standard for diagnosis, showing lobar consolidation or interstitial infiltrates depending on the pathogen. In CAP, lobar pneumonia is most common with S. pneumoniae, while interstitial patterns are seen with atypical pathogens.
    • CT Scan: Can provide greater detail, particularly in complicated or unclear cases.
  • Microbiological Testing:
    • Sputum Gram stain and culture: Can identify bacterial pathogens in sputum samples, although results may be unreliable due to contamination with oral flora.
    • Blood cultures: Recommended in patients with severe pneumonia or suspected bacteremia.
    • Urinary antigens: Useful for diagnosing Legionella and S. pneumoniae.
  • Laboratory Findings:
    • Elevated white blood cell count (leukocytosis): Common, particularly in bacterial pneumonia.
    • Procalcitonin: May be elevated in bacterial pneumonia and used to guide antibiotic therapy.
Management
Outpatient Management
  • Antibiotic Therapy:
    • Empiric therapy is based on the most likely pathogens and local resistance patterns. Initial therapy for uncomplicated outpatient CAP includes:
    • Amoxicillin or doxycycline for previously healthy adults with no risk factors.
    • Macrolides (e.g., azithromycin) or respiratory fluoroquinolones (e.g., levofloxacin) for patients with comorbidities or recent antibiotic use.
Inpatient Management
  • Empiric Therapy for Hospitalized Patients:
    • For patients requiring hospitalization for severe CAP:
    • Ceftriaxone or ampicillin-sulbactam plus a macrolide (azithromycin) or a fluoroquinolone (levofloxacin).
    • For severe CAP with suspected MRSA or Pseudomonas, add coverage with vancomycin or linezolid (for MRSA) and piperacillin-tazobactam or meropenem (for Pseudomonas).
  • Adjunctive Therapy:
    • Oxygen therapy: For hypoxic patients (SpO2 < 90%).
    • Fluids: For patients with dehydration or septic shock.
    • Corticosteroids: Not routinely used in CAP unless severe inflammatory response or concomitant septic shock.
Complications
  • Parapneumonic Effusion/Empyema:
    • Accumulation of pus in the pleural space, requiring drainage and extended antibiotic therapy.
  • Lung Abscess:
    • Cavitary lesion with necrosis of lung tissue, typically treated with prolonged antibiotics.
  • Sepsis and Septic Shock:
    • Pneumonia is a common cause of sepsis, especially in immunocompromised or critically ill patients.
  • ARDS (Acute Respiratory Distress Syndrome):
    • Severe lung injury causing hypoxemia, requiring mechanical ventilation in severe cases.
Prevention
  • Vaccination:
    • Pneumococcal vaccine (PCV13 and PPSV23) is recommended for adults ≥65 years and younger patients with comorbidities.
    • Influenza vaccine: Annual vaccination is recommended to prevent viral pneumonia and secondary bacterial infections.
  • Smoking cessation:
    • Smoking cessation significantly reduces the risk of pneumonia.
Key Points
  • Pneumonia is an infection of the lung parenchyma, most commonly caused by S. pneumoniae in CAP. Other pathogens include H. influenzae, M. pneumoniae, Legionella, and respiratory viruses (e.g., influenza, RSV).
  • Diagnosis is based on clinical features (fever, cough, dyspnea) and confirmed by chest X-ray showing lobar consolidation or diffuse infiltrates.
  • Empiric antibiotic therapy is chosen based on the clinical setting and severity, with outpatient therapy typically including amoxicillin or doxycycline and inpatient therapy including ceftriaxone or fluoroquinolones.
  • Complications include parapneumonic effusion, lung abscess, and sepsis. Prevention includes pneumococcal and influenza vaccination.

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